1. Field of the Invention
The present invention relates generally to an automotive keyless entry system which allows locking and unlocking of vehicular lock devices, such as door lock device, trunk lid lock device, steering lock device and so forth, utilizing a pocket-portable radio transmitter which transmits a radio signal containing a preset code. More specifically, the invention relates to an antenna device for radio signal communication between the pocket- portable transmitter and a controller mounted on a vehicle. In more detail, the invention relates to an antenna device which prevents the pocket-portable transmitter from being locked in the vehicle.
2. DESCRIPTION OF THE BACKGROUND ART
The copending U.S. patent application Ser. No. 651,783, filed on Sept. 18, 1984, by Motoki HIRANO and assigned to the common assignee to the present invention. discloses an automotive keyless entry system which prevents a radio code signal transmitter from being locked in the vehicle. In the disclosed system, a controller detects locking of a door lock device and repeatedly and cyclically transmits a demand signal to the transmitter for triggering the latter to transmit a radio code signal. An alarm is triggered if the radio code signal from the transmitter continues for a given first period of time. If the radio code signal from the transmitter continues for a given second period of time after the alarm, the keyless entry system is disabled. The first period of time is long enough for a user to move away from the vehicle out of radio signal transmission range. The second period of time is determined to be long enough to allow the user to unlock the door and remove the transmitter from the vehicle. Disabling of the keyless entry system ensures theft-prevention, since otherwise the door lock device could be easily unlocked by triggering the controller by depressing a push-button mounted on the outer surface of the vehicle.
In addition, the co-pending U.S. Pat. Application Ser. No. 651,784, now abandoned filed on Sept. 18, 1986, by Motoki HIRANO and assigned to the common assignee to the present invention, discloses an antenna device suitable for radio signal transmission between the transmitter and the controller by way of electromagnetic induction between antennas of the transmitter and the controller. The proposed antenna device is mounted on a vehicle and contains two loop antennas offset in phase by approximately 90.degree. to ensure radio signal transmission to and from the antenna on a transmitter. Therefore, this prior proposed antenna device ensures radio signal transmission with a limited radio signal transmission range.
Limiting the radio transmission area prevents the transmitter and/or the controller from being unintentionally triggered due to noise in the atmosphere.
However, if the transmitter should be left within the vehicle but out of the radio transmission area, it would be impossible to notify the user that the transmitter is locked in the vehicle.